In aviation, an aeroplane engine requires regular maintenance and a very rigorous inspection regime in order to ensure total reliability of the engine in operation. So that the internal parts as well as any wear thereof can be monitored, holes are provided in the engine case. During maintenance operations, a plug sealing the hole is unscrewed and a borescope can be inserted to probe the internal voids and inspect the condition of the engine parts. This avoids opening the case and completely dismantling the engine.
The commonly used plugs of this type, also called “borescope plugs”, have a screw thread and are screwed into a corresponding thread in the case, using a torque wrench. When the engine runs, its operation generates vibration capable of causing screwed parts, and more particularly the borescope plugs, to loosen.
Various means for avoiding such a fault are known. These means can consist of placing an insert in contact with the plug, before or after tightening. The parts used are of the lock washer type, or the split pin or lockwire type.
However, the plugs in an engine can be poorly accessible, because tubing and other peripheral components occupy the space around the case. Thus, installing an additional braking or locking part onto a plug requires great dexterity, and even luck, to avoid losing said part. In some cases, the plug is only accessible using a long tool, any direct manual access to the plug being impossible when the plug is in place. A plug equipped with a braking system dealing with this type of problem is also known from document PCT/FR95/01461. Such a system comprises locking and unlocking means that are automatically activated by the rotation of the plug and its longitudinal movement along the axis of rotation. The locking and unlocking means are constituted by two rings (or cups) each comprising a series of teeth directed axially towards the other ring. The teeth have a pointed (or triangular) profile and mesh together during the axial movement of the plug. One of the rings is fixed on the case, in the document press-fitted onto the case so as to be immobilized against rotation with respect to the case. The other ring is incorporated into the plug and driven in rotation therewith when the plug is being screwed into the hole in the case. This known plug requires the fitting of a ring onto the case, by press-fitting. This operation requires a special tool equipment.
It is noted particularly that the coupling for common rotation of one of the rings with the central body of the plug is obtained by mutually corresponding configurations made on the one hand on the ring and on the other hand on the central body of the plug. In the device described in said document, the ring comprises pins entering axially with respect to the direction of screwing into corresponding grooves configured on the central body of the plug, said pins being movable along said longitudinal axis. In a variant, similar devices are known in which the ring is directly coupled for common rotation with the central body by a system of flat sections. These systems all require specific machining of the central body, which can in certain cases weaken it mechanically and/or be unsuitable in terms of achieving a compact (small space requirement) and optimized plug.
The sealing devices of the type discussed are often exposed to severe operating conditions, such as extreme temperatures, moisture, dust etc., and are nevertheless expected to have the same lifetime as the equipment, typically an aircraft, on which they are mounted, i.e. 20 years or more, corresponding to several tens of screwing and unscrewing operations. The above-discussed sliding arrangements of the ring may sometimes become detrimentally frictional over the time.
In certain known sealing devices, the plug head has a face adapted to transmit tightening thrust to a bearing zone of the case when the plug is in its tightened condition in the threaded hole of the case. This however generates a frictional contact of the plug against the case around the threaded hole at the end of screwing and at the beginning of unscrewing. This may locally damage the case generally made of a relatively delicate alloy.